In recent years, increase of energy demand, depletion of fossil fuel such as petroleum, coal, and natural gas as energy sources, and emission of greenhouse gas such as CO2 are big social problems. Meanwhile, in electric power generation using fossil fuel, two thirds of primary energy supply is not utilized and discharged as thermal energy and further the two thirds of that is distributed widely as low temperature heat of lower than 200° C. It is desired to convert such low temperature heat into electric energy efficiently in order to obtain much energy from limited fossil fuel.
As a method for directly converting thermal energy into electric energy, there is a thermoelectric conversion technology using Seebeck effect. The thermoelectric conversion technology can convert energy without emitting a greenhouse gas and can be applied even in the low temperature region of lower than 200° C. Electric power generation that uses unused low temperature heat and causes a small impact on the environment therefore is expected to be obtained by using the thermoelectric conversion technology.
There is a figure-of-merit Z as an index showing the performance of a thermoelectric conversion material. Since a figure-of-merit Z has the dimension of [K−1], a dimensionless figure-of-merit ZT obtained by multiplying Z by an average temperature T and represented by the following expression (1) is used. T is an average temperature (absolute temperature), S is a Seebeck coefficient, p is an electric resistivity, and κ is a thermal conductivity.
                    ZT        =                                            S              2                        ρκ                    ⁢          T                                    (        1        )            
As a thermoelectric conversion material having an excellent dimensionless figure-of-merit ZT in a low temperature region of lower than 200° C., a full-Heusler alloy represented by an Fe2VAl type alloy is known for example. Such a full-Heusler alloy: comprises elements that are non-toxic, low-priced, and abundantly reserved; and attracts attention in recent years also from the viewpoint of impact on the environment. A full-Heusler alloy has a large Seebeck coefficient S. A full-Heusler alloy has a high thermal conductivity κ and a high electric resistivity p however and hence a dimensionless figure-of-merit ZT cannot be increased to a practical level.
As a measure against the problem, in PTL 1, a method of obtaining a high dimensionless figure-of-merit ZT by reducing a thermal conductivity κ is studied. Specifically, a complex thermoelectric conversion material having a thermal conductivity lowered by complexing and sintering a full-Heusler alloy and an additive of a low thermal conductivity is disclosed. As a method for producing such a thermoelectric conversion material, disclosed is a method of: alloying a thermoelectric conversion material having a composition represented by the general expression (Fe1-xMx)2V1-yLyAl1-zRz (in the expression, M is at least one element selected from the group of Co, Ni, Pd, Ir, and Pt, L is at least one element selected from the group of Ti, Cr, Mn, Zr, and Mo, R is at least one element selected from the group of Mg, Si, P, S, Ca, Ge, Sn, Sb, and Bi, and the expressions 0≤x≤0.2, 0≤y≤0.2, and 0≤z≤0.2 are satisfied) by mechanical alloying; successively mixing Bi as an additive of a low thermal conductivity; and applying electric current sintering to them under pressure. The disclosure says that, by the method, a material of a low thermal conductivity can be dispersed evenly and finely in a structure and resultantly a thermoelectric conversion material having a high thermoelectric conversion efficiency is obtained.
Further, in PTL 2, a full-Heusler alloy of a Fe2TiSi type is disclosed. Specifically, disclosed is a thermoelectric conversion material that is represented by the composition formula Fe2+σTi1+ySi1+z and has values of σ, y, and z allowing the thermoelectric conversion material to fall within a region surrounded by points (50, 37, 13), (50, 14, 36), (45, 30, 25), (39.5, 25, 35.5), (54, 21, 25), and (55.5, 25, 19.5) {excluding (50, 25, 25)} in terms of (Fe, Ti, Si) in at % in an Fe—Ti—Si ternary alloy phase diagram.